Crystals structures of carbonate phases with Mg in Triassic rocks, mineral formation and transitions

Detailed data on carbonate minerals different in magnesium content including their crystal structures were presented in this article. The samples were collected from the formations of the Lower Muschelkalk and the Upper Muschelkalk. X-ray diffraction and electron microprobe analysis were used for carbonate phases determination. The following minerals with different Mg content were identified: low-Mg calcite, high-Mg calcite, proto-dolomite, ordered dolomite and de-huntite. The content of Mg in high-Mg calcite is higher than in low-Mg calcite but lower than that of proto-dolomite. Proto-dolomite is characterized by lower content of Mg than typical for stoichiometric value for dolomite—ordered dolomite. Due to the reduced Mg content in identified huntite, this carbonate phase was named as de-huntite. The research results are significant because they are a source of new data on carbonate phases with different Mg content which build studied Triassic rocks and crystal structures of these minerals.

of carbonate phases.The results of study also allow to understand the carbonates' phases transitions during sedimentary processes and diagenesis.
X-Ray Diffraction and Electron microprobe analysis were used for identification of mineral phases.X-ray diffraction of limestones was carried out at the Department of Applied Geology in Gliwice using diffractometer HZG4 with a copper lamp and a nickel screen and the following analysis conditions: voltage 35 kV, intensity 18 mA.The method of reflective light was used here.Measurements were executed with a range of d-spacing (Å) from 0.8563 to 0.0953 nm.Carbonate phases in dolomites were identified by X-ray diffraction (XRD) carried out in the laboratory of the Solid State Department, Institute of Physics, University of Silesia using the EMPIRIAN diffractometer by PANALITYCAL.X-ray microanalysis was carried out at the Institute of Non-ferrous Metals in Gliwice.The analyses were conducted using the techniques of X-ray microanalysis EPMA, with application of a JXA-8230 X-ray microanalyser manufactured by JOEL.The examinations were performed on polished sections which were sputtered with a carbon coat.The analysis with the application of WDS spectrometers was carried out in micro-areas of all samples.The WDS method was applied to conduct quantitative analyses in micro-areas, in selected points having different chemical compositions.The content of the following chemical elements was determined: Mg, Si, Al, Ca, Ba, Sr, Fe, Mn, as well as the content of O and C.

Results of X-ray diffraction
The results of X-Ray Diffraction were in Fig. 2. The following minerals were identified in limestones: low-Mg calcite, high-Mg calcite, proto-dolomite, ordered dolomite, huntite, quartz and orthoclase.In dolomites protodolomite, ordered dolomite, high-Mg calcite and quartz were determined.
According to the results generally low-Mg calcite dominates in limestones (Fig. 2a-c).There are also higher amounts of high-Mg calcite, proto-dolomite and ordered dolomite.In samples S2 and SO14 huntite was identified (Fig. 2a,c).Small contents of quartz occur in sample SO14 (Fig. 2c) and orthoclase in samples S2 and SO14 (Fig. 2a,c).The results of X-ray diffraction indicate that sample S2 is typical limestone but the samples TO62 and SO14 are probably dolomitic limestones.In dolomite samples proto-dolomite and ordered dolomite dominate (Fig. 2d,e).There are also higher contents of high-Mg calcite and small amounts of quartz.Therefore these rocks are typical dolomites.
Due to the diversity of the sample S2 6 (Terebratula Unit), microprobe measurements were made in two microareas of this sample.The research of the results indicate that the content of MgO in huntite in this limestone is lower than the stoichiometric value for this carbonate phase (MgO-34.25%)(Fig. 3a,b, Table 1).Probably some magnesium was removed from crystals during dehuntization processes.Also thermal decomposition of huntite into aragonite is possible.In one sample of Karchowice Unit-TO7 5 two calcite phases were identified: low-Mg calcite and high-Mg calcite.Moreover ordered dolomite was determined (Fig. 3c, Tables 2).In samples TO62 5 , SO14 6,8 (Fig. 3d,e, Table 2) low-Mg calcite dominate.Moreover ordered dolomite was identified and, in sample TO62 10 -proto-dolomite (Fig. 3d, Tables 2), but in sample SO14 6,8 (Fig. 3e, Table 3)-huntite.As in sample S2, also in sample SO14 huntite is characterized by a reduced content of magnesium in relation to the stoichiometric value for this mineral phase (MgO-34.25%)(Fig. 3e, Table 3).
Slight variation in the amount of mineral phases was observed in the dolomite samples of Tarnowice Unit.Only high-Mg calcite and proto-dolomite were determined in samples LZ1 and PSK2 (Fig. 3f,g, Table 4).Variable Mg content in high-Mg calcite and lowered MgO amount in proto-dolomite were observed.This may indicate that they could be calcareous dolomites.

Summary of research results
The results of X-ray diffraction and Electron microprobe analysis indicate the presence of five carbonate phases varied in Mg content: low-Mg calcite, high-Mg calcite, proto-dolomite, ordered dolomite and huntite.
The results of X-Ray diffraction indicate that with the increase of magnesium in the crystal the values of diffraction lines decrease 43,44 .Summarizing the results of X-ray diffraction it can be concluded that sample S2 is typical limestone and the samples TO62 and SO14 are dolomitic limestones.The samples LZ1 and PSK2 are typical dolomites.
In BSE images low-Mg calcite and high-Mg calcite are mixed in limestone rock mass (Fig. 3e,f).The content of Mg in low-Mg calcite is below 3% (MgCO 3 amount is below 10.75%).The content of Mg in high-Mg calcite ranges from 6.69 to 10.70% (MgCO 3 value ranges from 23.84 to 38.34%).Proto-dolomite is characterized by Mg content from 11.12 to 12.61% (MgCO 3 value ranges from 39.65 to 45.18%).In ordered dolomite Mg content ranges from 13.02 to 13.67%.(MgCO 3 value ranges from 46.65 to 48.98%).Therefore in proto-dolomite the MgCO 3 value is lower than the stoichiometric one for this carbonate phase.In ordered dolomite is similar

Discussion
In the Triassic (Lower Muschelkalk) limestones from the area of Opole Silesia, five carbonate phases were identified: low-Mg calcite, high-Mg calcite, proto-dolomite, ordered dolomite and huntite.Two carbonate phases were identified in the Triassic (Upper Muschelkalk) dolomites from the area of Upper Silesia: high-Mg calcite and proto-dolomite.
On the basis of the results the chemical formulas of identified carbonate phases were calculated and probable crystal structures of these minerals were created.The results were compared with the data obtained by other scientists presented in their works 6,29,35,[45][46][47][48][49][50][51][52][53] .The crystal structures of the carbonate phases were prepared on the basis of data from references 6,29,35,[45][46][47][48] .Preparing the frameworks of crystal structures, the Mg content shown in the calculated chemical formulas was included.The research results allow to determine cell parameters of identified carbonate phases including the data from previous studies 26,35 (Table 5).It was also possible to determine the appropriate space group of identified mineral phases comparing the obtained data with information from the studies of other scientists 26,35 (Table 5).
According to the results low-Mg calcite (Fig. 4a, Table 5) was determined in the Lower Muschelkalk limestones but not in Upper Muschelkalk dolomites.High-Mg calcite (Fig. 4b, Table 5) determined in limestones of Lower Muschelkalk and dolomites of Upper Muschelkalk is characterized by differences in Mg content.In a single crystal cell of high-Mg calcite which is built of 14 ions three of them could be Mg ions and 11-Ca ions [5][6][7]9 . Wih the increase of MgCO 3 content in high-Mg calcite the values of cell parameters-a o (Å) and c o (Å) drop 40,54 .If the amount of MgCO 3 is 22.7% high-Mg calcite present the cell parameters as follows: a = 4.91 Ǻ, c = 16.65 Ǻ and for the amount of MgCO 3 36.71%their values are lower: a = 4.88 Ǻ, c = 16.45Ǻ 40,54 .Magnesium substitutions in high-Mg calcite make the structure of crystal cell different in comparison with low-Mg calcite.This is related to the difference in the size of the ionic radii of Ca and Mg 37,38,54,55 .The symmetry of high-Mg calcite crystal is rhombohedral (space group R3c) similar to the typical for proto-dolomite (Table 5) 26,35 .Two dolomite phases were determined: proto-dolomite and ordered dolomite.Proto-dolomite is characterized by lower content of magnesium than ordered dolomite, but higher than high-Mg calcite.Moreover, the MgO content in protodolomite is lower than the stoichiometric value for dolomite (MgO-21.86%,Mg-13.18%).The symmetry of  proto-dolomite crystal (Fig. 4c, Table 5) is rhombohedral with the space group R3c 27,36,46 , similar to the typical one for high-Mg calcite.The space group of ordered dolomite (Fig. 4d, Table 5) is rhombohedral-R3 but a little bit different to that of proto-dolomite 27,36 .The content of MgO is similar to stoichiometric value for this carbonate phase.In the Triassic limestones of the Polish part of the Germanic Basin (Opole Silesia) also huntite was identified 6,7,9,47 .However this huntite presents lower value of MgCO 3 (50.20% to 57.98%) than the typical one for stoichiometric value for this carbonate phase, which ranges from 69.30 to 72.28%.The reduction of Mg in this mineral can be an effect of diagenetic processes-dehuntization (calcitization?)6,7,9,47 .This phase was named as de-huntite.On the basis of determined chemical formula and crystal structure of huntite with stoichiometric magnesium content (Fig. 4e(A)) 29,47 theoretical structure of de-huntite was drawn up (Fig. 4e(B), Table 5).
According to the results low-Mg calcite and high-Mg calcite were probably formed in the epicontinental Germanic Basin during direct crystallization from sea water, at the same time as aragonite and dolomite phases.Moreover, owing to the presence of waters of the phreatic zone and salty sea waters which had an elevated Table 2. Microprobe chemical analyses in the micro-area of samples TO7 5 and TO62 5 .

Point number/mineral chemical formula
Micro-area of sample TO7-type of chemical element [%mass] (Fig. 3c content of dissolved mineral components, the fixation process of unstable high-Mg calcite originally formed in the environment of the seabed took place 6,7,9 .High-Mg calcite and aragonite are an unstable phases which undergo transition into low-Mg calcite.Only the presence of Sr and Ba in calcite indicate that the primary phase was aragonite.High-Mg calcite was probably preserved in Triassic rocks during early diagenetic processes.Dolomite phases were formed in the mixing zone of the waters from the phreatic zone and salty sea waters in the early stage of constructive diagenesis during dolomitization processes.In this environment proto-dolomite and ordered dolomite could have been formed.However the transition of proto-dolomite into ordered dolomite during advanced stages of diagenesis is possible.Huntite of sedimentary rocks is formed in the vadose zone 6,7,9,17 .De-huntite of the Germanic Basin was probably formed in the areas in which diagenetic processes were taking place with the contribution of waters from the vadose zone.It could be the reason for the reduced content of magnesium in this mineral.Transition process connected with reducing magnesium ions from huntite crystals can be described as dehuntization (calcitization?).The research results were compared with the others' works in the context of previous works including world data.It has been done to increase the scientific significance of this article.According to the data from previous studies the content of Mg in low-Mg calcite varies from 0.00 to 3.00% 1,2,9, 12,18,19,55 .The research results show that in Triassic limestones the Mg content ranges from 0.06 to 3.49%.These values are therefore comparable to the data obtained by other scientists.The low-Mg calcite crystal structure was prepared on the basis of data presented by Packet and Reader 35 and Maslen et al. 45 .It is the theoretical structure of a low-Mg calcite crystal with no Mg ions in crystal structure.The content of Mg in high-Mg calcite varies generally from 7.00 to about 11.00% 9,12,[18][19][20][21][22][23][24][25][26]32 . Trissic include high magnesium calcite with amount of Mg from 6.73 to 10.70%.Therefore, the research results are similar to the data from previous studies.In the previous work, a hypothetical structure of magnesium calcite was prepared, in which the arrangement of magnesium ions in the crystal was proposed 6 .It was based on the models proposed by Wenk et al. and Tsipursky and Buseck 6,37,56 .Wenk et al. 56 found that magnesium calcite has a ν-type structure.In this structure, in addition to layers containing calcium ions, there will also be layers containing Ca and Mg ions along the [100] direction, which gives a calcium to magnesium ratio of 3:1 (Ca:Mg = 3:1) 6,37,56 .According to the information from references 19,22,31,40,42,48 proto-dolomite is nonstoichiometric, poorly ordered dolomite phase.It is characterized by a reduced Mg content compared to the stoichiometric value of dolomite [18][19][20]26,31,40 .Proto-dolomite was identified in one limestone sample TO62 (from Karchowice Unit) and in dolomite samples (from Tarnowice Unit).Proto-dolomite of limestone is characterized   by Mg content from 11.85 to 12.61%.Mg content in proto-dolomite of dolomites ranges from 11.12 to 12.98%.These values are higher than typical for high-Mg calcite but lower to stoichiometric for dolomite. Poto-dolomite is treated by some scientist as a phase formed from high-magnesium calcite.It is carbonate transitional phase between magnesian calcite and ordered dolomite 31,40,48 .High-Mg calcite is characterized by 0-32 mol% of Mg substitution for Ca. Proto-dlomite has about 55-60 mol% of Ca in the lattice with incomplete segregation of Ca and Mg into separate layers 48 .Stoichiometric, ordered dolomite is characterized by 50:50 of Ca to Mg ratio with the near perfect ordering of the Mg and Ca in alternate cation layers 48 . Th proto-dolomite crystal structure was prepared on the basis of the ordered dolomite crystal structure, include the reduced number of magnesium ions determined in proto-dolomite.Ordered dolomite is characterized by stoichiometric value of Mg for this carbonate phase (MgO-21.86%,Mg-13.18%)[18][19][20]22,26,48,51,52 .Mg content in ordered dolomite of Triassic limestones ranges from 13.20 to 13.67%.Therefore the measured contents are slightly higher than the stoichiometric value for dolomite.The ordered dolomite crystal structure was prepared on the basis of the data presented by Antao et al. 46 and Mehmood 48 .In the ordered dolomite crystal structure the amount of Ca and Mg ions is the same.Huntite is carbonate phase characterized by higher Mg content (20.65% of Mg, 71.92% of MgCO 3 ) than in ordered dolomite 17,[27][28][29][30] .However, the determined Mg content in huntite of Triassic limestones is definitely lower than stoichiometric one.It ranges from 14.01 to 15.92%.That's why this carbonate phase was named as de-huntite. The redced content of Mg in this mineral can be an effect of dehuntization 6,7,9,47 . De-huntite crysta structure was prepared on the basis of the structure of huntite presented by Dollase and Reeder in their work 29 .Creating de-huntite crystal structure, a reduced amount of Mg ions was included.

Conclusions
Five carbonate phases were determined in Triassic limestones and dolomites of Polish Part of Germanic Basin: low-Mg calcite, high-Mg calcite, proto-dolomite, ordered dolomite and de-huntite.Low-Mg calcite and dehuntite occur only in Triassic limestones.In dolomites only high-Mg calcite was identified.Moreover Triassic dolomites are characterized by presence of proto-dolomite and lack of ordered dolomite and de-huntite.
The content of Mg in high-Mg calcite is higher than in low-Mg calcite but lower than in proto-dolomite.Proto-dolomite is characterized by lower Mg content than typical for stoichiometric one for dolomite.Ordered dolomite present Mg content similar to the stoichiometric value.De-huntite presents lower value of MgCO 3 than typical for stoichiometric one for huntite.The reduction of Mg in de-huntite can be an effect of diagenetic process, dehuntization.On the basis of the results chemical formulas of identified carbonate phases were calculated and crystal structures of these minerals were prepared including study results and reference data.Research results indicate the similarity of the geochemical composition of Triassic limestones from the Opole Silesia and dolomites from Upper Silesia which include carbonate phases with different Mg content with the world data on Triassic carbonate rocks presented in the references.
The data allowed to form the theory about possible formation of identified carbonate phases and diagenetic processes that influenced their current structure and preservation of unstable high-Mg calcite.Low-Mg calcite and high-Mg calcite were formed during direct crystallization from sea water, at the same time as aragonite and dolomites.High-Mg calcite was preserved in Triassic rocks during diagenetic processes.Owing to the presence of the phreatic zone waters and salty sea waters also the fixation process of high-Mg calcite could take place.Dolomite phases were formed in the mixing zone of the waters from the phreatic zone and salty sea waters during dolomitization processes.But the transition of proto-dolomite into ordered dolomite during advanced stages of diagenesis is possible.De-huntite was formed in the areas of Germanic Basin where diagenetic processes were taking place with the contribution of waters from the vadose zone.
To obtain new data on carbonate phases with different Mg content, the Triassic limestones of Opole Silesia and the Triassic dolomites of Upper Silesia will be subjected to further analyses.Especially two methods will be used: X-Ray Fluorescence (XRF) and Fourier Transform Infrared Spectroscopy (FTiR).They will provide new data on the presence of carbonate phases with different Mg amount in the studied Triassic rocks and the range of Mg content in these phases.
https://doi.org/10.1038/s41598-023-46013-2www.nature.com/scientificreports/ to stoichiometric.Mg content in huntite ranges from 14.01 to 16.18% and MgCO 3 value ranges from 50.20 to 57.98%.So as it is lower than stoichiometric value for this carbonate phase which varies from 69.30 to 72.28% of MgCO 3 .The reduction of Mg in huntite could be an effect of diagenetic processes (dehuntization/calcitization?)6,7,9 or thermal decomposition of huntite into aragonite45 .The results of microprobe measurements indicate that samples S2, TO7, TO62 and SO14 represent dolomitic limestones with different content of carbonate phases varied in Mg amount.The samples LZ1 and PSK2 represent typical dolomites including proto-dolomite and high-Mg calcite.

Table 1 .
Microprobe chemical analyses in the micro-areas of sample S2 6 .

Table 3 .
) Microprobe chemical analyses in the micro-area of sample SO14 6 .

Table 4 .
Microprobe chemical analyses in the micro-areas of samples LZ1 and PSK2.

Table 5 .
Structures of crystal cell and chemical formulas of carbonate phases with magnesium.